Liquid volatile organic compounds (“VOC”) are stored and dispensed from tanks. A very common example is in the field of distribution of combustion engine fuel such as gasoline for fueling automobile and aircraft engines. The storage tanks usually have large capacities, receive bulk shipments of fuel from a supply source and dispense smaller amounts in multiple events, e.g., filling individual automobile tanks at service stations. The gas space above the liquid in the tank is sometimes called the “ullage” of the tank. Usually there is a high concentration of VOC in the ullage of fuel storage tanks.
Prior to the time that air pollution by VOC emissions became an environmental protection concern, emission controls on storage tanks were directed mainly to preventing fire and explosion hazards. Few controls were aimed at curbing fugitive emissions such as emissions of VOC incidental to dispensing fuel from bulk storage tanks and to storing the fuel in the tanks.
More recently heightened awareness has developed of the need to reduce fugitive emissions resulting from storage and dispensing of VOC. As a result vapor recovery systems for VOC with increasing degrees of sophistication have been deployed. For example, to reduce environmental emissions of VOC vapor during automotive and other types of fuel fill-ups, fuel suppliers and distributors have begun to install vapor recovery systems at fuel dispensing stations. Such systems usually have suction equipment that draws VOC vapors and air present during fuel transfer at the fuel dispensing nozzle back to the ullage of a bulk storage tank. The returning gas mixture enters the void in the tank created when the dispensed liquid leaves.
Traditionally storage tanks merely had P/V valves (Pressure-Vacuum valves) intended to maintain the tank within a range of slight positive and negative pressure, i.e., a few inches of water pressure. The returning gas mixture from dispensing operations, as well as other factors, caused pressure to build up in the ullage over time. Of course, when tank pressure exceeded the upper limit of the P/V valve, excess gas containing VOC was discharged to the environment.
Certain advanced VOC fugitive emission control systems are designed to operate with a slight negative pressure in the ullage of the bulk storage tank. That is the tank is under a vacuum relative to ambient atmosphere. Such systems offer the advantage that any leaks that occur will cause outside air to flow into the vapor recovery systems, rather than allow vapor to escape to the atmosphere. In addition to the gas buildup mentioned earlier, air in-leakage contributes to pressure increase in the tank. The liquid fuel evaporates into the incoming fresh air and the mass of the vaporized fuel plus the mass of air within the fixed ullage volume increases the pressure. Negative pressure thus can only be maintained if gas is exhausted to the environment from time to time. However, it is necessary to strip all or a portion of the VOC from the exhausted gas. Otherwise, the VOC in the discharged gas defeats the purpose of the pollution control system.
Various techniques have been proposed to remove VOC emissions from bulk storage tanks operating at subatmospheric pressure. A method gaining commercial acceptance uses a selectively gas permeable membrane to separate the VOC component from the benign air component of the ullage mixture. The non-VOC component, composed primarily of nitrogen and oxygen, is preferentially permeable through the membrane and is emitted to atmosphere substantially free of the VOC component. VOC is less permeable, largely does not pass through the membrane and is returned to the storage tank.
The membrane separation vapor recovery system is contemplated to operate cyclically and emit to atmosphere discontinuously. Emissions occur only when the tank pressure exceeds a pre-selected high pressure limit. At other times, flow through the membrane is stopped. For example, tank pressure descends below the high pressure limit as a consequence of discharging primarily non-VOC component gas to the ambient atmosphere. At a preselected low pressure limit, discharge stops. At these times, the vapor is stagnant in the separation membrane module and in the gas transfer lines immediately upstream and downstream of the module.
Although the separation membrane selectively permeates oxygen and nitrogen, it does not absolutely reject VOC compounds. Consequently, the gas that permeates the membrane and is vented to the environment includes some VOC vapor, albeit less than that which would vent had the membrane not been utilized. It has been discovered that a very high concentration pulse of VOC vapor emits from the membrane module at the start of a venting cycle, i.e., directly after rising tank pressure initiates flow through the membrane and venting commences at the end of a stagnant period. After a while, the concentration of VOC in the permeate/exhaust gas decreases to a steady state value in the expected manner. A significant quantity of VOC vapor is released to the atmosphere by the time the gas venting portion of the cycle stops. As a result, the time-averaged quantity of VOC compounds discharged to the air is still unacceptably high.
It is desirable to reduce overall emissions of VOC compounds below that which results from conventional separation membrane-based, fuel tank vapor recovery systems. In U.S. pat. No. 6,719,824 there is disclosed a cyclic membrane separation process that is effective to reduce the time-averaged quantity of VOC compounds emitted to the environment. That process includes the steps of temporarily stopping flow to and from the membrane separator and adding a diluent gas, preferably ambient air, to the membrane separator while the flows are stopped. The diluent gas flows into the membrane separator via a blower or pressure gradient due to the typically slightly lower-than-ambient pressure in the system. The introduction of diluent air is thought to purge the membrane separator of excessive VOC such that the amount of VOC exhausted in the next cycle is lower. The present invention relates to a process for reducing VOC emissions in which excessive VOC is purged by drawing a vacuum on the membrane separator as will be more fully explained below.